1. Field of the Invention
The present disclosure relates generally to pipelined signal converters.
2. Description of the Related Art
Pipelined analog-to-digital signal converter systems are often used in high-speed, high-resolution conversion applications. These systems generally realize a desired number of conversion bits with a cascade (i.e., a pipeline) of lower-resolution converter stages and thus achieve high resolution at sampling speeds that are difficult to realize with other converter systems. Each stage of a pipelined system quantizes that stage's input signal to a predetermined number of digital bits and forms an analog output signal which is presented to a succeeding stage for further signal processing.
The advantages of sampling speed may, however, be negated if conversion linearity is insufficient. For example, the multistage structure of pipelined converter systems causes certain portions of the converter structure to be used repetitively as an analog input signal is swept over the system's input range and converter nonlinearity in these portions can significantly degrade the conversion of low-level dynamic signals.
Conversion linearity is generally characterized with a variety of linearity parameters such as differential nonlinearity (DNL), integral nonlinearity (INL), signal-to-noise ratio (SNR), signal-to-noise-and-distortion ratio (SINAD), and spurious free dynamic range (SFDR). DNL error indicates the difference between an actual step width of a least-significant bit and the ideal value while INL error measures the deviation of an actual transfer function from a straight line. SNR□ is computed by taking the ratio of the rms signal to the rms noise wherein the noise includes all spectral components minus the fundamental, the first four harmonics, and the DC offset. SINAD is the ratio (in dB) of the signal power to the power of all spectral components minus the fundamental and the DC offset. Finally, SFDR is the ratio of the fundamental component to the rms value of the next-largest spurious component (excluding DC offset).
Although a variety of linearizing techniques have been proposed for pipelined converter systems, increasing demands on these systems continue to exert a need for further improvements in linearity.